The objectives are to clarify selected issues related to the causation and classification of affective disorders (manic-depressive illnesses). The primary focus is on the genetic basis for bipolar and unipolar forms of depressive illness. The research setting is the Old Order Amish community of Lancaster County, Pennsylvania. This is a homogeneous, endogamous (in-bred) group, where total genealogical assessment is possible, making it uniquely suited for identification and control of genetic variables. There is full ascertainment (period prevalence) of cases of mental disturbances for the three-year period, 1977-1979. A Board of five psychiatrists provides independent diagnoses of the clinical case materials, using the Research Diagnostic Criteria. Confirmed instances of primary affective disorders serve as probands for the bipolar and unipolar samples. The present emphasis is on SADS-L interviewing of all first degree relatives of bipolar probands to develop extended pedigrees with multiple cases of illness. Several of these informative pedigrees will be selected for linkage analysis by studying blood types, color blindness and other known genetic loci.